Considerations of Observational Scale when Evaluating the Effect of, and Remediation
Strategies for, a Fluvial Tailings Deposit in the Upper Arkansas River Basin,
Colorado

ABSTRACT

We examined the water-quality effects of a fluvial tailings deposit along the
flood plain of the upper Arkansas River south of Leadville, Colorado. Fluvial
tailings deposits are a possible diffuse source of acid and metal contamination
to surface and ground water. We used four different scales of observation to
evaluate the potential effect of fluvial tailings on water quality. First, we
collected surficial material and subjected it to batch water-leaching tests.
Second, we excavated an intact 8-inch-diameter (60 centimeters in length) core,
leached it under unsaturated conditions for 23 days, and collected the effluent.
Third, we examined the water quality of the shallow ground water beneath the
fluvial tailings deposit; and fourth, we monitored water quality along a 5-kilometer
reach of the adjacent Arkansas River. Our results illustrate the importance
of observational scale in the interpretation of the effect of the fluvial tailings
deposit on water quality. Leaching of surficial samples indicates that there
is a large reservoir of readily water-soluble material yielding elevated metal
concentrations and high acidity that could degrade water quality. However, the
river-water-quality data indicate that there is no measurable effect from the
fluvial tailings deposit. It is important to note that this data set does not
include any stormwater sampling. Natural attenuation processes (including dilution)
appear to contribute to our different findings at different observational scales.
Attention to the importance of observational scale can lead to informed remediation
decisions.